Stabilizing the wavelength of a laser or light source is important in many applications. One such application is a telecommunication system that employs wavelength division multiplexing, in which a plurality of lasers or light sources each emit signals having different wavelengths that are multiplexed over a single fiber-optic cable. In this application, the plurality of signals having different wavelengths are demultiplexed after transmission across the fiber-optic cable and are delivered to corresponding recipients.
Due to the increased demands placed on telecommunication systems, it is increasingly desirable that fiber-optic cables employed in a wavelength division multiplexing system be capable of handling a large number of signals. Unfortunately, increasing the number of signals handled by a fiber-optic cable requires that the wavelengths of the different signals be closer together, which in turn increases the likelihood that the wavelengths of the signals will wander, and interfere with each other.
Additionally, it has been recognized that, over long periods of usage, laser and light sources gradually experience wavelength drift. In the short term, differences in temperature at a source can also cause fluctuations in the wavelength of the emitted light from a laser. Thus, in order to minimize the likelihood of wavelength crosstalk, it is necessary to stabilize the wavelengths of telecommunications transmitters.
Several methods have been devised for measuring and stabilizing wavelengths. FIG. 1 is a diagram that illustrates a prior art apparatus employing one such method, as disclosed in International Publication No. WO 97/05679 of PCT Application No. PCT/CA96/00416, published Feb. 13, 1997. In the figure, a Fabry-Perot etalon filter 14a receives light emitted from source 10, and based upon the wavelength of the light received, outputs a signal to detector 16a. The intensity of the signal detected by detector 16a is fed to circuit 11. Circuit 11 is also fed the intensity of the light detected by detector 16b, which receives a reference signal from source 10 via beam splitter 12. Circuit 11 compares the signals that it receives from detectors 16a and 16b, and adjusts the wavelength produced by source 10 if the ratio of the intensities of the two received signals changes beyond pre-determined limits.
FIG. 2 is a diagram that illustrates another prior art method, as disclosed in B. Villeneuve, H. Kim, M. Cyr and D. Gariepy, A Compact Wavelength Stabilization Scheme for Telecommunication Transmitters, Ontario, Canada. In the figure, source 10 emits light having a frequency which is received by photodetectors 16a and 16b via Fabry-Perot (hereinafter "FP") filter 14. The angular behavior of FP filter 14 (which varies the wavelengths of light received depending on the cosine of its tilt angle) results in differing spectral responses at photodetectors 16a and 16b. The differing spectral responses are converted into a discrimination signal 18, which is utilized to stabilize the wavelength.
These and other prior art methods (such as the use of holographic crystals, which are not shown) require the use of optical elements that are neither compact nor simple to implement. Furthermore, these and other prior art methods are not programmable or adjustable for various desired frequency channels.
Thus, there is a need for an improved apparatus for stabilizing the wavelength of a laser.